Electrolytic precipitation of uranium values from carbonate leach liquors



QUORS P 29, 1953 R. KUNIN ELECTROLYTIC PRECIPITATION OF URANIUM VALUES FROM CARBONATE LEACI-I LI Filed April 12, 1954 Permselecfive Mem bra ne IN VEN TOR. ROBERT KUNI N ATTOR NE United tes ELECTROLYTIC PRECIPITATION OF URANIUM VALUES FROM CARBONATE LEACH LIQUORS Application April 12, 1954, Serial No. 422,311

10 Claims. (Cl. 204-15) This invention relates to the recovery of uranium values. uranium oxide is precipitated and is isolated from carbonate leach liquors.

The methods currently in use for the precipitation of uranium from carbonate leach solutions or leach liquors all suffer from one or more serious disadvantages. Among the disadvantages are the cost of reagents, the production of gelatinous precipitates which are most difficult to separate, and the cost of replenishing the carbonate-bicarbonate leaching solutions.

The object of this invention is to treat a carbonate leach liquor so as to convert the uranium values into the form of an insoluble oxide which can be readily isolated and ignited to the oxide, U Another object is to replenish the carbonate-bicarbonate leaching solutions.

This object is accomplished efficiently and economically by a method of electrolysis wherein a direct electric current is passed through an electrolysis cell which is divided by means of a permselective membrane into a cathode compartment containing an aqueous solution of a uranyl salt and a water-soluble inorganic carbonate and a water-soluble bicarbonate, such as a carbonate leach liquor, and an anode compartment containing a water-soluble inorganic carbonate, preferably sodium carbonate.

The electrolytic cell which is employed can best be understood by a reference to the attached drawing wherein the single figure is a diagrammatical representation of a typical cell which is divided into an anode compartment and a cathode compartment by means of a permselective membrane or diaphragm. In said figure, 1 represents a container which is divided into two compartments, 5 and 6, by a permselective membrane, 2, which is described in greater detail below. Compartment 5 is an anode compartment by virtue of the presence there of the anode 3, while compartment 6 is a cathode compartment because it contains the cathode 4. When the cell is in operation, the electrodes are connected to a source of electric power not shown.

The cell which is employed in this invention can be varied as to size, shape, volumes of the individual compartments, vents, ports, exits, construction materials, controls, embellishments, means for admitting and removing the contents of the compartments, et cetera Without departing from the spirit of this invention. What is essential is that the cell have two compartments, one containing the anode and the other containing the cathode, and that the two compartments be separated by a perm selective membrane as defined herein.

The permselective membranes which divide the elec-.

trolysis cell into the two compartments are essential to the success of this process. They function by allowing It relates to an electrolytic process wherein atent 1 tirely suitable-for use in leaching more ore. more, because the catholyte contains carbonate ions, it

ice

ment to the other. Cationic permselective membranes are those which allow the passage of cations while anionic permselective membranes are those which permit the passage of anions. The membranes contain ion exchange resins and it is the presence of the ion exchange resins which imparts the property of permselectivity. While the composition of the permselective membranes can vary within reasonable limits, it is important that they contain enough ion exchange resin as to have suitably high conductance when employed in the electrolysis cell. The permselective films which have proven to be most suitable for use in this process are those made by incorporating particles of an ion exchange resin in a film-forming matrix such as polyethylene or vinyl resins. Such membranes are available commercially and are described in U. S. patent applications Serial Nos. 202,577, now Patent No. 2,681,320, and 205,413, now Patent No. 2,681,319, and in the corresponding Canadian Patents Nos. 493,562 and 493,563.

In the process of this invention, a leach liquor containing dissolved uranium values is placed in the cathode compartment 'of the electrolysis cell. The anolyte which is employed is ordinarily a solution of sodium carbonate but can be a solution of any water-soluble carbonate.

When a cationic permselective membrane is employed and as electrolysis proceeds,'sodium ions migrate toward the cathode. They are free to pass from the anode compartment through the cationic membrane into the cathode compartment. Hydrogen is liberated at the cathode and the catholyte becomes increasingly alkaline. Hydroxyl ions are formed and bicarbonate ions are converted to carbonate ions. The uranium is partially reduced and the pH gradually rises until at a pH above about 12 uranium precipitates as an oxide. This precipitate is very dense and can be most conveniently separated by conventional means such as filtration, decantation, or centrifuging and later ignited to U 0 It is far easier to handle and isolate than are the gelatinous precipitates obtained by other and older methods. At the same time oxygen is liberated at the anode, the anolyte becomes more acidic and carbonate ions are converted to bicarbonate ions. The net result is that uranium is obtained in a most convenient form, the catholyte is changed to a carbonate solution while the concentration of bicarbonate ions is increased in the anolyte.

When an anionic permselective membrane is employed, the current is carried for the most part by the anions which are free to pass from the cathode compartment through the anionic membrane into the anode compartment.

In any case, whether the membrane be anionic or cationic, the electromechanical changes which take place in the two compartments during electrolysis are the same. The precipitation of uranium oxide is the same. Also, the change of bicarbonate ions to carbonate ions in the cathode compartment and the change of carbonate; ions to bicarbonate ions in the anode compartment are the same with either kind of membrane.

The resultant advantage in the isolation of uranium is obvious. But another advantage is also gained; namely, that the anolyte which finally contains both carbonate and bicarbonate is of such a composition that it isencan be used as the anolyte in the electrolysis of the next batch of pregnant leach liquor.

Actually, the electrolytic process described above can be combined with the operation of leaching uranium ores in a cyclic or continuous process. For example, a pregnant leach liquor is run into the cathode compartment of the electrolysis cell described above through which is Patented Apr. 29, 1 958 Furtherpassing a direct current. .As the uranium precipitates, the I Example 1 A solution containing uranyl ions, sodium carbonate, andsodium bicarbonate was employed as a catholyte in the electrolytic process described above. An aqueous solution :of sodium carbonate was employed as the anolyte. The cells were constructed of polymethylmethacrylate (Plexiglas). Both electrodes were platinum, although carbon, graphite, and copper are also quite satisfactory as cathodes and graphite or lead coated with lead oxide are also satisfactory as anodes. The cells were divided into anode and cathode compartments by means of permselective membranes (Amberplex A1 and Cl) known to contain about 70% ion exchange resin in amatrix of polyethylene and known to have been made according to the process of U. S. patent applications Serial Nos. 202,577 and 205,413 and the corresponding Canadian Patents Nos. 493,562 and 493,563 of June 9, 1953. The cationic membranes contained a cation exchange resin which was a sulfonated cross-linked copolymer of styrene and divinylbenzene While the anionic membranes contained an anion exchange resin made by aminating with a tertiary amine a cross-linked, chloromethylated copolymer of styrene and divinylbenzene. I

During electrolysis the anolytes and catholytes were mechanically agitated. When the electrolysis was complete, the catholyte was filtered and the filtrate was analyzed for uranium. The anolyte was titrated with standard acid solution to determine the bicarbonate'coucentration. The anolyte was also analyzed for uranium but in every case the amount of uranium was considered negligible.

The solution which was employed contained 2.1 grams of uranium (measured as U 100 grams of sodium carbonate, and 20 grams of sodium bicarbonate in each liter. The anolyte was a one molar solution of sodium carbonate.

Following is a tabulation of the results of several tests. The column headed Volume is a measure in milliliters of the individual volumes of the cathode and anode compartments of the electrolysis cell. Also Kwh. is a measure of the kilowatt-hours of electricity consumed, while percent U pptd is a measure of the percentage of uranium precipitated during the individual runs from each batch of the original leach liquor. The Current density represents the number of amperes per square foot of area of the permselective membrane in the cell.

Current Volume, Percent Bun Membrane Density, ml. Kwh. U pptd amps/sq. ft.

Anionic--- 87 140 0.022 98 Cationic 87 140 O. 013 93 d0 65 140 0. 016

- Anionic- 91 140 0. 022 97 Cationic 85 1, 000 0. 121 90 can be readily explained. Thus, when the -cell contains an anionic membrane, the current is carried by the transport of carbonate ions. If, however, any significant concentration of hydroxyl ions exists in the catholyte, they too migrate to the anode compartment. Indeed, since hydroxyl ions have a higher transport number than carbonate ions, they may carry a proportionately larger amount of the current. This reduces the pH in the catholyte and causes conversion of some of 'the bicarbonate ions to carbonate ions in the anolyte. The total effect is to increase the cost of precipitating the uranium. On the other hand, when, a cell is divided intoanode and cathode compartments by means of a cationic membrane, the hydroxyl ions which are formed in the cathode compartment are prevented from migrating into theanode compartment by the cationic membrane; and the current is carried almost exclusively by the positive ionsin this case, the sodium ions.

Example 2 In another series of tests the same procedure was followed as is described in Example 1. The pregnant liquor which was employed was somewhat difi'erent'in that it was a 0.9 molar solution of sodium carbonate and a 0.1 molar solution of sodium bicarbonate and also contained 3.3 grams of uranium per liter. The results of the tests are here tabulated:

Current Volume, Percent Run Membrane Density, m1. Kwh U ppt'd amps/sq. ft.

Cationic- 38 140 0.011 64 While the average ampere efliciency in this series was 87%, it is also apparent, as might be expected, that the cost of precipitating uranium increases as'the concentration of the uranyl ions decreases. Therefore, there may be some instances where it might be more economical to precipitate only part of the uranium and then recycle the resultant dilute catholyte together with more of the concentrated pregnant liquor. The chief disadvantage of this procedure is that the catholyte would always contain some uranium and, therefore, would not be used as the anolyte in the electrolytic process, as was described above.

Example 3 In this case the electrolysis cell was made of polymethylmethacrylate (Plexiglas). The anode and cathode compartments each had a volume of one gallon and were equipped with stirrers. The same cationic permselective membrane (Amberplex C1) described above was usedto divide the cell into the anode and cathode compartments. The anode was lead and the cathode graphite. The anolyte was a one molar solution of sodium carbonate while the catholyte was a carbonate leach liquor fromM'onticello, Utah, containing 1.5 grams/liter of ,uranium and 9 grams/liter of vanadium. A direct current was passed through the cell, at a'current density of '90 'amperes'per square foot of area of the permselective diaphragm, for one hour. Uranium oxide precipitated in the cathode compartment and was removed by filtration. Analysis showed that 92% of the uraniumvalues was thus removed.

, I claim:

1. A process for the isolation of uranium values,-which comprises passing a'direct electric current through-an electrolysis cellcomprising 'a' cathode compartment having therein an aqueous solution which is made up of a carbonate and a bicarbonate of an alkali metal and a uranyl salt, and an anode compartment having therein anaqueous solution which is made up of a carbonate of an alkali metal, said cathode and anodecompartments being separated by a permselective membrane which contains an ion exchange resin, continuing the passage of current 5 until the pH of the catholyte is above about 12.0 and until an insoluble uranium oxide has precipitated and thereafter separating said precipitated uranium compound from said catholyte.

2. The process of claim 1 in which the permselective membrane contains an anion exchange resin.

3. The process of claim 1 in which the permselective membrane contains a cation exchange resin.

4. The process of claim 1 additionally including the step in which the uranium oxide which is precipitated and separated from the catholyte is ignited to U 5. A process for the isolation of uranium values, which comprises passing a direct electric current through an electrolysis cell comprising a cathode compartment having therein an aqueous solution which is made up of a uranyl salt, sodium carbonate and sodium bicarbonate, and an anode compartment having therein an aqueous solution which is made up of sodium carbonate, said cathode and anode compartments being separated by a permselective membrane which contains an ion exchange resin, continuing the passage of current until the pH of the catholyte is above about 12.0 and until an insoluble uranium oxide has precipitated, and thereafter separating said precipitated uranium oxide from said catholyte.

6. The process of claim 5 in which the permselective membrane contains an anion exchange resin.

7. The process of claim 5 in which the permselective membrane contains a cation exchange resin.

8. The process of claim 5 additionally including the step in which the uranium oxide which is precipitated and separated from the catholyte is ignited to U 0 9. A cyclic process for isolating uranium values from uranium-bearing ores which comprises first leaching said ores with an aqueous solution of a carbonate and a bicarbonate of an alkali metal; electrolyzing the resultant leach liquor in the cathode compartment of an electrolysis cell which contains a cathode compartment and an anode compartment separated by a permselective membrane containing an ion exchange resin, said anode compartment containing an aqueous solution of the same alkali metal carbonate which is present in said leach liquor; continuing the electrolysis until the pH of the catholyte is above about 12.0 and until an insoluble uranium oxide has precipitated; removing said uranium precipitate and transferring the remaining solution to the anode compartment of an electrolysis cell which is divided into an anode compartment and a cathode compartment by means of a permselective membrane containing an ion exchange resin; removing the anolyte from said first electrolysis cell and employing it in the leaching of more uraniumbearing ore; and repeating the electrolysis operation and the distribution of the products of the electrolysis operation.

10. The process of claim 9 in which the leaching solution is a solution of sodium carbonate and sodium bi carbonate.

References Cited in the file of this patent UNITED STATES PATENTS 1,448,036 Pearson et a1 Mar. 13, 1923 2,581,863 Kahn Jan. 8, 1952 2,606,148 Portanova et a1. Aug. 5, 1952 2,681,319 Bodamer June 15, 1954 2,681,320 Bodamer June 15, 1954 2,743,222 Clevenger Apr. 24, 1956 FOREIGN PATENTS 493,562 Canada June 9, 1953 493,563 Canada June 9, 1953' OTHER REFERENCES Analytical Chemistry of the Manhattan Project, C. I. Rodden, 1st ed., McGraw-Hill Book Co., New York (1950), page 8.

The Chemistry of Uranium, Part I, by Joseph J. Katz and Eugene Rabinowitch, 1st ed., McGraw-Hill Book Co., New York (1951), page 118. 

1. A PROCESS FOR THE ISOLATION OF URANIUM VALUES, WHICH COMPRISES PASSING A DIRECT ELECTRIC CURRENT THROUGH AN ELECTROLYSIS CELL COMPRISING A CATHODE COMPARTMENT HAVING THEREIN AN AQUEOUS SOLUTION WHICH IS MADE UP OF A CARBONATE AND A BICARBONATE OF AN ALKALI METAL AND A URANYL SALT, AND AN ANODE COMPARTMENT HAVING THEREIN AN AQUEOUS SOLUTION WHICH ID MADE UP OF A CARBONATE OF AN ALKALI METAL, SAID CATHODE AND ANODE COMPARTMENTS BEING SEPARATED BY A PERMSELECTIVE MEMBRANE WHICH CONTAINS AN ION EXCHANGE RESIN, CONTINUING THE PASSAGE OF CURRENT UNTIL THE PH OF THE CATHOLYTE IS ABOVE ABOUT 12.0 AND UNTIL AN ISOLUBLE URANIUM SAID PRECIPITATED URANIUM COMPOUND THEREAFTER SEPARATING SAID PRECIPITATED URANIUM COMPOUND, FROM SAID CATRHOLYTE. 